docs: def get_data_filename(filename):
      """Map filename to its actual path.
  
      Parameters
      ----------
      filename : str
          Filename to search.
  
      Returns
      -------
      path : str
          Full path to the file in data directory.
  
      """
      global _data_map
  
      if _data_map is None:
          _data_map = {}
          for root, dirs, files in os.walk(specdir):
              for fname in files:
                  _data_map[fname] = os.path.join(root, fname)
  
      if filename not in _data_map:
          raise KeyError(filename + ' not found in ' + specdir)
      return _data_map[filename]